In general, a shield tunneling method for continuously constructing a circular tunnel by boring the tunnel by a shield tunneling machine and assembling segments in a ring shape inside the shield tunneling machine is known as a method for constructing a tunnel. When the shield tunneling machine assembles the segments in the ring shape in the tunnel while excavating in this shield tunneling method, it is necessary to grasp the assembly error (roundness) or the shape of an existing segment and modify the assembly of a subsequent segment. Thus, in general, various methods have been proposed as a method for grasping the shape or the roundness of the existing segment, as disclosed in Japanese Patent No. 2820011, for example.
In Japanese Patent No. 2820011, there is disclosed an existing segment shape grasping method for grasping the shape of an existing segment (segment ring) by measuring distances to the inner peripheral surface of the segment by a plurality of sensors provided in an erector device. Specifically, in this existing segment shape grasping method, the coordinates of a plurality of positions on the inner peripheral surface of the segment are obtained on the basis of the distances to the inner peripheral surface of the segment measured by the plurality of sensors and the turning angles of the sensors, and the shape of the existing segment (segment ring) is grasped by approximating the shape of the existing segment by an ellipse on the basis of eccentricity between the center of the segment and the turning centers of the sensors (the turning center of the erector device).